Defer schedule catchup window default to server

[chaptersix]

What changed

• Change the default SchedulePolicy.catchup_window from an SDK-defined year to None.
• Omit the protobuf field when it is unset.
• Preserve absent fields as None when decoding.
• Add serialization and server-backed default coverage.
• Update the changelog.

Why

The server owns the default. Materializing one year in the SDK prevents server-side default changes from applying consistently.

Breaking changes

An unconfigured policy now exposes None before it is sent to the server. Described schedules continue to expose the server-resolved duration.

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[CLAassistant]

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[tconley1428]

Seems like this needs a breaking change notification.

[chaptersix]

hmm, on seconds thought. I think it's fine to for an SDK to default to a year.